About 200 million years ago (MYA), the Tethys Sea split Pangea into Laurasia (the northern continents) and Gondwanaland (the southern continents); the latter included Africa, India, Antarctica, South America and Australia. Since that time, the Indian Ocean has formed as these Southern Continents split from one another.
In the mid Jurassic Period, 150 MYA, the Atlantic Ocean began to form, splitting Europe from North America and Africa from South America. By the early Cretaceous Period, some 130 MYA, a landmass including Madagascar, the Seychelle Islands and India rifted from the rest of Gondwanaland and, by 100 MYA, Africa split from Antarctica as the Southwest Indian Ridge began to form. About 85 MYA, India rifted from the Madagascar-Seychelles land mass and Australia began to split from Antarctica along the Southeast Indian Ridge. Finally, the Mid Indian Ridge (which trends NNW to SSE and connects the Southwest and Southeast Indian Ridges) began spreading as well, accelerating India's movement toward southern Asia; its northwestern extension, known as the Carlsberg Ridge and ending in the Gulf of Aden, began to open about 62 MYA and India finally slammed into Asia 55 MYA, lifting the Himalayas (a process that continues today).
As these four spreading zones continued to open, Africa drifted to the NNW, Antarctica moved southward, India plowed northeastward into Asia and Australia drifted eastward and then northeastward, all surrounding the vast Indian Ocean which has an average depth of 12,900 feet; the portion of the Indian Ocean south of the Southwest and Southeast Indian Ridges is often referred to as the Southern Ocean. While the Carlsberg Ridge is no longer active, the Southwest, Mid and Southeast Indian Ridges continue to produce oceanic crust; in concert, the African Plate continues to move NNW (lifting the Alps and igniting volcanism in southern Europe) and Australia is drifting NNE. Most evident are the earthquakes and subduction volcanoes along the western and southern rims of Indonesia, where the Australia Plate dips below the Eurasian Plate.